Previously, aluminium was not widely used for tool making, however, due to improvements in the aluminium properties, and particularly the mechanical properties of aluminium, it is now well known to use aluminium for tool making. In molding, and especially in plastic and polymer molding, aluminium has been widely used for prototyping molds as the molds are cheaper and faster to manufacture than steel molds, and further weigh much less making them suitable for prototype molding.
The aluminium molds are typically coated by chrome or nickel plating, or they may be anodized. These coatings protect the aluminium molds against both corrosion and wear and tear. Furthermore, the coatings increase the abrasion resistance of the molds. Also, a PVD process may be used to deposit for example TiN, TiC, etc. Steel is still the predominant material for forming molds designed for high volume production, and the steel molds are typically coated with tungsten carbide or silicon nitride, typically in layers of 5 μm to 200 μm.
To obtain the corrosion and abrasion resistance necessary for the aluminium molds even in prototyping molds, an efficient coating needs to be provided. The typical coating layer thickness of the coatings as mentioned above, ranges from 5 μm to 200 μm, where PVD coatings may allow for the thinnest coatings, though also being a very complicated process.
These thick coatings may act as thermal barriers, and may, thus, limit the heat flux through the mold surfaces, slowing the heat exchange and prolonging cycle time. Furthermore, these coatings typically have different coefficient of thermal expansion compared to the bulk of the mold, which results in stressing and straining of the interface which may result in failures, such as delamination or flaking during thermal stressing of the mold. Additionally, if the coating is damaged, repair of the surface is difficult as the coating is neither readily repaired nor readily removable, so that often the mold will be discarded and not repaired.
It has been known to use aluminum molds for stamping, such as for imprinting, such as for nano-imprinting. and it is known e.g. from US 2005/0039618 to use an aluminium stamp for transferring a pattern in nano-scale, wherein the aluminum stamp has a silane based monomolecular anti-stiction coating. However, as the stamping process is much less harsh to the molds than e.g. injection molding, typically as disclosed in e.g. US 2003/0080458, aluminum molds for molding of liquid materials such as PUR have, when they have been employed, used a brushing or spraying technique to apply anti-stiction coating in between molding cycles. Such techniques may be adequate for some objects, however, the precision of the layer thickness, etc. will be insufficient for other purposes and furthermore also it may be a challenge to ensure that the mold is tightly closed during molding when a release agent is applied by brushing or spraying.
Furthermore, US2011/0139959 discloses a method of preparing an aluminium mold by providing a chemically bonded silane based anti-stiction coating effective for molding of thermoplastic parts. However, even this silane based coating does not result in a significantly higher lifetime for the mold, and frequent re-coating of the mold is required to ensure the anti-stiction properties of the mold.
Thus, there is a need for an improved coating for aluminium molds.